I was able to track down some specific details for two examples. No idea if this info helps, but here it is.
Mil Lilies in Ultimate Caves have a base DAR of 100% and RDR of 87.5% for the Greenill section ID. On Hardcore, there is a minimum RDR boost of 30%. So 1.3 * 0.875 = 1.1375 or over 100%. Therefore, in theory, these Mil Lilies should always drop a DB's Saber (3062) for Hardcore players. Yet, I have a text from my friend on 12/27/2019 who told me that he had found a Mil Lily that dropped meseta instead. He wasn't too worried about it, but he thought it was weird given there shouldn't have been any chance of that happening.
On a separate occasion, I was farming for a Devil/Technique from Pouilly Slimes in VHard Caves on Redria. The DAR and RDR in this case is exactly the same as for the Mil Lilies I described before. On 2/8/2020, I texted my friend to let him know that the Pouilly Slime I found dropped me a Mind Material. Needless to say, I was a bit more upset than my friend given this is actually what I was farming for.
My friend seemed to think it must be the DAR boost that's affecting rare drops in some way. Personally I don't agree, but it does seem to indicate that there was a DAR boost active in both cases.